This is caused by missing phrases. Specifically, we have code like this which is going wrong:

PHP:

$post['month'] = new XenForo_Phrase('month_'.$post['month'].'_short'); // used for XenPorta, falls back to month_00_shortcase 'M': $output .= $phrases['month_' . $month . '_short']; continue; // used in XenForo itself, the phrase isn't in the array so we get nothing appended to the string which removes the month

For whatever reason, the phrases seem to have been deleted on this site. For the AM/PM issue, it would appear to be the same cause: